1. Field of the Invention
The present invention relates to a spark plug for internal combustion engine having a better self-cleaning function in use of surface creeping spark discharges.
2. Description of Related Art
Recently, as the environmental preservation has been watched with more keen interest, stratified charge internal combustion engines with lower fuel consumption have been widely noticed as environment-friendly engines.
However, when stratified fuel mixtures are burned in a combustion chamber, rich fuel mixtures are concentrated near a spark slug so that the spark plug may tend to be smoldered or fouled by carbon. The carbon-fouling makes an insulation property of an insulator surrounding a center electrode worse so that a spark discharge may not occur across a regular discharge gap provided between center and ground electrodes but occur between the insulator, on a surface of which carbon is deposited, and an inside of a metal housing for mounting at a portion deep into the metal housing from a front end surface of the insulator.
To cope with this problem, there are known self-cleaning spark plugs as disclosed in JP-Y2-53-41629 or JP-A-4719236.
According to JP-Y2-53-41629, the spark plug has a plurality of electrodes constituting first and second ground electrodes. A first discharge gap is formed between the first ground electrode and the center electrode and a second discharge gap is formed between the second ground electrode and the center electrode. A regular spark discharge occurs through the first discharge gap and, when the insulator is fouled by carbon deposit, a spark discharge occurs through the second discharge gap, not through the portion deep into the metal housing, so that carbon may be burned without decreasing ignitability of the spark plug.
Further, according to JP-A-47-19236, there are provided with the regular first discharge gap and the second discharge gap through which sparks are discharged when the insulator is fouled. It is characterized, in this case, that a front end of the center electrode is nearly equal in height to a front end of the insulator.
Therefore, as the spark discharge at the first discharge gap occurs at a position nearly same in height as the second discharge gap, it is contemplated, therefore, that the respective ignitability characteristics at both first and second discharge gaps do not have much difference.
However, the spark plug according to JP-Y2-4719236 has a drawback that there exists a big difference of ignitability between the respective spark discharges at the first and second discharge gaps, since the second discharge gap formed at a leading end of the metal housing is arranged at a position far away from the first discharge gap, so that drivability is adversely affected, in particular, in the stratified fuel combustion.
Further, as the spark discharge at the second discharge gap occurs at a place deep from the leading end of the insulator into an insulator base, channeling is likely to occur. on the other hand, according to the spark plug disclosed in JP-A-4719236, there is also a problem that ignitability is not good, as the front end of the first electrode is obliged to be almost same in height as the front end of the insulator so that the front end of the insulator may operate to cool flame cores generated by spark discharge at the first discharge gap.